The functions of switch units, and more particularly those forming circuit breakers, include the provision of a safe breaking function for, in the event of a short-circuit, protecting both consumers and installations. Electrical or mechanical switch units are also suitable for the in-service manual switching of consumers, and for the safe isolation of an installation from the power grid for conduct of maintenance work or modifications to the installation. Electrical switch units are often electromagnetically operated.
Switch units of this type are accordingly highly-engineered electric switching devices with integrated protection for motors, cables, transformers and generators. They are employed in functional locations where a low frequency of switching is required. In addition to short-circuit protection, switch units of this type are also suitable for providing overload protection.
In the event of a short-circuit, an electrical switch unit safely disconnects an electrical installation. Fuse protection against overload is also thereby provided. Every conductor in which an electric current flows will heat up, to a greater or lesser extent. Such heating-up is dependent on the ratio of the current rating to the conductor cross-section, i.e., the “current density”. The current density must not be excessively high, or excessive heat-up would otherwise result in charring of the conductor insulation or the possible start of a fire. In order to protect electrical installations against these damaging effects, switch units are employed as overcurrent protection devices.
Circuit breakers are provided with two independently-acting release mechanisms for overload and short-circuit protection, connected in series. Short-circuit protection is assumed by a virtually instantaneously-acting electromagnetic release. In the event of a short-circuit, the electromagnetic release immediately releases a latching mechanism of the circuit breaker. A switching armature separates the contact piece before the short-circuit current can reach its maximum value.
Known switch units comprise a contact slide unit with a contact slide and a moveable contact piece. The moveable contact piece is furthermore provided with electrical contacts. Switch units of this type are additionally provided with first contacts to a current conductor. In its closed state, the electrical contacts of the moveable contact piece are in contact with the fixed contacts of the switch unit. In the event of a short-circuit, the electrical contacts of the moveable contact piece are released from the fixed contacts, thereby interrupting the flow of electric current. The moveable contact piece is thus released or disengaged from the fixed contacts. In the event of short-circuit tripping in a switch unit, however, the moveable contact piece may be caused to rotate about its longitudinal axis after the moveable contact piece is released. Where the moveable contact piece rotates about its longitudinal axis, it is designated a rotating bridge element. In other words, following its rotation, the moveable contact piece does not return to its original position, but instead remains in its rotated position.
In many cases, known contact slides of contact slide units are provided with two guide systems, namely, an internal guide system and an external guide system. The external guide system is employed where the switching process, i.e. the making or breaking operation, is executed by means of a latching mechanism of the switch unit. In this case, no rotating bridge element results. The internal guide system is employed in the event of a short-circuit, if the switching process is executed by means of a switching armature, generally a tappet, on the switch unit. This means that, in the event of disconnection in response to a short-circuit, the moveable contact piece precedes the contact slide along the internal guide system, rebounds against the impact surfaces provided in the lower part of the switch unit and is then propelled back along the internal guide system. Accordingly, it is propelled in the opposing direction relative to the switching armature or tappet of the switch unit. It is therefore possible that the moveable contact piece and the tappet engage outside their respective mid-lines, thereby resulting in the rotation of the moveable contact piece around its longitudinal axis.
Upon the next switching operation of the switch unit, if the moveable contact piece remains in its rotated position, the contacts, specifically silver contacts of the moveable contact piece, will no longer engage with the fixed contacts of the switch unit, thus resulting in malfunctions. This means that the contact piece remaining in a rotated position is disadvantageous, in that the switch unit will then no longer be serviceable. A non-functioning contact piece and a non-functioning switch unit are disadvantageous for both the electrical consumer and the installation in which the switch unit is installed.
A further problem arises where, in the event of a short-circuit, the circuit breaker does not sufficiently rapidly interrupt the short-circuit current. Three time-staggered contact-opening mechanisms are employed for this purpose. A short-term and transient opening of contacts in response to a current flow is effected, first, by the application of current loop forces on the contact position between the fixed and moveable contact pieces and, second, by means of a pin that is electromagnetically driven by the short-circuit release. The permanent opening of contacts is effected by a disengageable mechanical kinematic chain, in combination with a switching lever.
It is accordingly problematic that if such high short-circuit currents occur, the time sequence of the contact-opening mechanisms no longer functions. In this case, the very high current loop forces present result in correspondingly rapid acceleration of the moveable contact piece, which will then rebound so rapidly from the limit stop of the housing that the contact will be reclosed before the latching mechanism can be maintained in the permanently open position by means of the switching lever. The destruction of the device may then ensue.